1. Field of the Invention
This invention relates to a device for detecting a temperature distribution using backscattered light. More particularly, light backscattered from an optical fiber may be used to determine the temperature distribution along the fiber. Temperature measurements may be thus made along the entire length of the fiber without the disadvantages inherent in electrical sensors such as electrical interference and electrical sparking. The invention is suitable for use in any application where it is desired to know the temperature along any point on the optical fiber.
2. Description of the Background Art
The background of the invention may be understood by reference to the following publications and patents which are incorporated herein by reference:
(1) Kingsley, Stuart A., "Distributed Fiber-optic Sensors," PROC. OF THE 1984 ISA SHOW, Houston, Tex. 22/25 Oct. 1984, pp. 315-330;
(2) Dakin, J. P., et al., "Temperature Distribution Measurement Using Raman Ratio Thermometry," SPIE, Vol. 566, Fiber Optic and Laser Sensor III (1985) pp. 249-256;
(3) Hartog et al., U.S. Pat. 4,823,166; and
(4) Dakin, U.S. Pat. 4,673,299.
Optical fibers using backscattered light for temperature distribution measurements employ pulsed laser beams as the input light into the fiber. The light backscattered at each point along the fiber contains temperature data in the form of the spectra, intensity and polarization of the backscattered light. Optical Time Domain Reflectometry (OTDR) is utilized to measure and process the backscattered light. The backscattered light propagated back through the fiber is detected and processed as time series signals, and a unidimensional temperature distribution along the optical fiber is measured. The time of receipt of the backscattered light is proportional to the distance along the fiber at which the scattering occurred, which is related to the temperature at the point of scattering.
In general, three types of scattered light are produced: Rayleigh scattered light due to density fluctuations, Brillouin scattered light due to interactions with acoustic waves of similar wavelength as that of the propagating light, and Raman scattered light due to the vibration and rotation of molecules interacting with the propagating light.
While Rayleigh scattering is elastic, Brillouin and Raman scattering are inelastic, and produce spectrums which are different from the spectrum of the incident light.
While temperature data is included in all of the three types of scattered light, Raman scattered light is most sensitive to temperature.
Raman scattering involves the transfer of energy between the vibrational modes of the glass material in the fiber and the incident photons of the laser source. The Raman scattering may result in the creation of a photon (Stokes Raman scattering) or in the transfer of energy from a vibrational mode to an existing photon (anti-Stokes Raman scattering). Stokes Raman scattered light has a wavelength shifted to a longer wavelength as compared with the wavelength of the incident light and anti-Stokes Raman scattered light has a wavelength shifted to a shorter wavelength as compared with the wavelength of the incident light. The Stokes Raman and anti-Stokes Raman backscattered light are selected by a filter, and the temperature distribution is calculated using a value based on the ratio of the two backscattered and filtered beams. (See Ref. 2). However, it is possible to use only the anti-Stokes backscattered beam instead of the ratio, e.g., see Ref. 3.
The filter may be fabricated from a dielectric multilayer or a metallic layer. The scattered light is shifted several scores of a nano-meter, and the amount of shift is a function of the material composition of the fiber.
A problem in the prior systems is the failure to stabilize the wavelength of the laser light used as the incident pulse light with the filter cut-off frequency. When the relationship between the filter cut-off wavelength and the laser oscillation wavelength changes, inaccuracies occur in the temperature measurements along the fiber resulting in, for example, a shift of a zero point or a slope error.
To produce a commercially useful temperature distribution detecting device, it is desirable to ensure that the wavelength of the laser radiation incident on the fiber is kept constant and within the cut-off frequencies of the filter so as to ensure accurate temperature measurements.
However, in conventional temperature distribution detecting devices as described above, since the ratio of the intensity of the anti-Stokes Raman scattered light to the intensity of the Stokes Raman scattered light is a function essentially of temperature, it can not be determined whether a change in the detected signals is due to temperature variations or a change in the oscillation wavelength of the laser.